The temporary crown is a short-term prosthesis which has the intended function of restoring a diseased dental element from the time of preparation of a tooth to the time of sealing of the final prosthesis.
In odontology, the problem of temporary prostheses is one which gives rise to particular concern for at least two reasons. In the first place, it is necessary to meet the classic needs of patients who desire to conceal the unattractive appearance of prepared teeth during the period of construction of prostheses. In the second place, it is necessary to ensure that the temporary prosthesis provides pulp protection of the tooth which has been ground or permits endodontic treatment in the event of substantial decay, restores or maintains a sound periodontium, permits orthodontic displacement if necessary, locks the abutment tooth in its relationships with the opposing and adjacent teeth recorded during the denture impression, and guides the dental prosthetist during preparation of the final prosthesis.
It is already a known practice to construct temporary dental crowns by means of a ring which is placed on and around the previously ground tooth stump to be restored and which is filled with resin. Rings of known types are subject to certain drawbacks. Some rings are of metal, which is aesthetically objectionable, and they are strictly cylindrical, which makes them very unattractive. Preformed rings of polycarbonate are also known. These rings are attended by the following disadvantages: their occlusal faces are stereotyped without any possiblity of adaptation to the opposing teeth. Thus the shape of the temporary crowns obtained from rings of this type does not correspond to the shape of the teeth on which they are positioned. Before fitting them on the teeth, the dentist is consequently obliged to modify their shape in order to adjust them. In the majority of instances, the ring is frusto-conical and flared-out in the downward direction without taking into account the anatomy of a pitted tooth. Furthermore, the open end of the ring must be cut to a festoon shape in order to correspond more or less to the gum contour line.
The final crown is a prosthesis which is sealed or bonded in order to be permanent. The problem of attractive permanent dental crowns is becoming increasingly significant with the appearance of composite resins. In fact, it is possible with these materials to achieve complete restorations of teeth. By making use of amelodentinary adhesives, it is possible to bond these materials to the enamel and to the dentin of teeth.
It is already a known practice to carry out complete tooth restorations by means of a ring which is placed at a tooth location and filled with a composite resin of the type mentioned above. However, these rings are subject to certain disadvantages.
Metallic rings have a frusto-conical shape, which does not usually correspond exactly to the anatomical shape of the other teeth within the oral cavity. This difference in anatomy between a natural tooth and the crown thus formed, which is totally unattractive, may be the cause of poor food deflection which will result in lesions at the level of the marginal gum. Moreover, these metallic rings do not permit the use of photopolymerizable composites. A further point is that no satisfactory method exists for modifying the contour of these rings in order to make them more attractive and to adapt them to the shape of the tooth to be restored and of teeth which are adjacent to this latter.
Rings of celluloid are also known but their extreme deformability is such that they achieve only very imperfect reproduction of the real anatomy. Furthermore, it is difficult to grind them since a grinding operation has the immediate effect of cutting their thin wall.
A matrix for dental use is also known, as disclosed in French patent Application No. 2 454 795, and serves to facilitate the filling of lateral teeth. This matrix consists of a profiled strip having an arcuate transverse cross-section and curved so as to form a closed outwardly bulged ring having dimensions such as to permit approximate restoration of the anatomical shape of a tooth destroyed by dental caries or mechanical influences. This strip is provided with at least one tearing tag which serves to split the matrix into two or more sections so as to facilitate its removal after hardening of the tooth-filling product.
A matrix of this type, however, would not prove suitable for forming a temporary crown which is intended to remain within the oral cavity over a period of several weeks. There are a number of reasons for this: on the one hand, the tearing tags constitute an outward projection and could not be allowed to remain within the patient's mouth. On the second hand , the top edge of the strip forms a sharp projection which, in this case also, cannot be permitted to remain within the mouth. In addition, an effective seal between said edge and any possible filling material could not be maintained for a very long time. Finally, the external profile of the strip is not that of a tooth and is thus also unsuitable for the construction of a crown even of a temporary type.